This invention relates to an electric discharge machining method and apparatus for machining the longitudinal surfaces of a workpiece having thin-walled parts, like turbine blades.
Electric discharge machining has heretofore been used for machining workpieces having complex shapes because it is capable of forming hard materials of poor machinability into the same shape as that of electrodes. It has the advantage of being capable of high-precision machining.
FIG. 1 is a diagram of assistance in explaining the construction of typical electric discharge machining apparatus of a conventional type. In FIG. 1, numeral 51 refers to a bed, at the edge of which a column 52 is provided. A feed mechanism 53 provided at the upper end of the column 53 causes a holder 55 holding an electrode 54 to be disposed in a vertically movable manner. On the bed 51 provided is a machining tank 57 containing a working liquid 56, in which a workpiece 58 is placed. With the electrode 54 and the workpiece 58 connected to a power source (not shown), the electrode 54 is caused to lower by the feed mechanism 53 so that electric discharge is generated between the electrode 54 and the workpiece 58 in the working liquid 56 to form holes, recesses and other shapes in the workpiece 58.
FIG. 2A is a front view showing an example of a turbine blade as a workpiece; FIGS. 2B and 2C being cross-sectional views taken along lines A--A and B--B in FIG. 2A, respectively. In FIGS. 2A through 2C, a turbine blade 60 comprises a base 61 which is mounted on a turbine shaft, and a vane 62 which is formed into a broad, thin-walled section, whose side surfaces 64 and 65 along the major axis 63 have complex curvatures as shown in FIGS. 2B and 2C.
Since turbine blades 60 as shown in FIGS. 2A through 2C are usually made of a hard material, such as SUS630, for example, and their vanes 62 are formed into thin-walled sections, it is extremely difficult to machine their side surfaces 64 and 65.
Workpieces of hard materials such as mentioned above are normally formed by electrolytic machining. That is, with a turbine blade 60 being machined used as an anode and a cathode, both immersed in an alkaline electrolytic solution, like NaOH, a direct current is applied to effect electrolysis to remove the metal of the turbine blade 60. In such a case, electrolytic machining is carried out by injecting the electrolyte under high pressure to prevent metallic ions from adhering on the cathode. Since the cathodic electrode is hardly consumed, the turbine blade 60 as a workpiece can be formed into the exact contour of the electrode. Furthermore, heat or external force never affects the workpiece, producing no degraded layers on the surface of the workpiece.
Electrolytic machining, however, has a difficulty in determining the contour of the electrode. In electrolytic machining where machining is effected with a jet stream of electrolytic solution, the flow of electrolytic solution tends to be changed by the initial shape of the workpiece, making it difficult to maintain machining accuracy. To overcome this, the shape of electrode has to be determined through a process of several or several dozens of trials and errors. For this reason, electrolytic machining is suitable for long-run mass production of uniform products, but not suitable for short-run production of variations in sizes and grades.
Electric discharge machining, on the other hand, can produce a workpiece that is 100-200 .mu.m smaller than the electrode at any portions facing the electrode. An effective means for discharge machining a workpiece, such as a turbine blade 60, into a predetermined shape is to use electrodes having surfaces corresponding to the blade surfaces 64 and 65.
It is necessary, however, to remove minute cracks on the discharge-machined surfaces after machining. An effective means to remove such cracks is to carry out electrolytic machining or electrolytic grinding by flowing electrolytic solution on the machined workpiece after discharge machining, with the electrode disposed close to the workpiece.
During electric discharge machining, however, heat is generated on the surface of the workpiece due to discharge between the electrode and the workpiece, causing the surface of the workpiece to expand in the direction along the surface. When the side surfaces 64 and 65 of the turbine blade 60 as shown in FIGS. 2A through 2C are discharge machined separately, the surface of the turbine blade 60 tends to expand in the direction along the side surfaces 64 and 65, or in the direction along the major axis 63, causing the vane 62 to warp due to the small wall thickness of the vane 62, leading to lowered dimensional accuracy. The larger the size of the major axis 63 of the turbine blade 60 and the smaller the wall thickness of the vane 62, the more pronounced becomes this tendency.